Radio Astronomers Reveal "Bizarre" Behavior of Gamma-Ray Burster

Radio astronomers today revealed that the first gamma-ray burster ever
detected at radio wavelengths has surprised them by its erratic
behavior. "We expected the burster to act at radio wavelengths much as
it does at X-ray and visible wavelengths -- that is, rise in
brightness, then slowly become weaker," said Dr. Dale Frail of the
National Radio Astronomy Observatory (NRAO). "Instead, it has
completely surprised us." The announcement was made at the American
Astronomical Society meeting in Winston-Salem, NC.

Frail and Dr. Greg Taylor, also of NRAO, used the National Science
Foundation's Very Large Array (VLA) and Very Long Baseline Array
(VLBA) radio telescopes to study the gamma ray burst which exploded on
May 8. The VLA was turned on the region of the burst within four hours
of its discovery by the new orbiting Italian-Dutch observatory,
Beppo-SAX. The radio astronomers have observed the object on a
near-daily basis since then, and that monitoring has revealed the
burster's unexpected behavior.

"It is remaining at a roughly steady level of brightness, but has
occasional flares in which it brightens by factors of two to three,"
Frail said. These variations are seen not only by the VLA, but also
by the VLBA, a continent-wide radio telescope system that shows the
object to be less than a few light years in size. The VLBA
observations show a level of detail more than 50 times greater than
that of the Hubble Space Telescope at optical wavelengths.

For years, positions of gamma-ray bursts were known only to within
several degrees in the sky, because of the limitations of earlier
gamma-ray telescopes. Using the VLBA, the astronomers now have
pinpointed a position for the May 8 burst to within a thousandth of a
second of arc. "In only a year, this field of research has progressed
to the point that we have a position more than a million times more
accurate than before," Taylor said. This has allowed the researchers
to show that the burster has not moved in the month since its
discovery.

What causes the unexpected flares in the radio output of this object?
"One idea is that we are seeing the radio analogy of stars twinkling
in the night sky," Frail said. This would require the object to be
very small. Another idea is that the variations in the radio emission
are not connected to the burst itself, but are arising within an
active galaxy in which the burst occurred.

"In order to solve the 30-year mystery of what causes gamma-ray
bursts, we need to know what kind of galaxies give rise to them,"
Frail said. Both the VLA and VLBA will continue to observe this
burster to gain additional information about its behavior in the
coming months. "Theorists have made detailed predictions about how the
radio intensity of such objects should behave over time. The ongoing
VLA monitoring will test those predictions. Similarly, the great
resolving power of the VLBA may allow us to track changes in the
structure of the object over time."

Gamma ray bursts were first discovered in the 1960s, and have
mystefied scientists since then. Because the positions of these bursts
could not be well determined before the launch of BeppoSAX, it was
difficult for optical and radio astronomers to follow up on the
gamma-ray detections. A longstanding controversy arose over whether
the explosions creating these powerful bursts of radiation are
occuring within our own Milky Way Galaxy or in other galaxies perhaps
billions of light-years distant. Optical studies of the May 8 burst
indicate that it is at least 7 billion light-years away, thus
apparently resolving the distance question. The exact nature ot the
explosions is still a question for further study.

The VLA and VLBA are instruments of the National Radio Astronomy
Observatory, a facility of the National Science Foundation, operated
under cooperative agreement by Associated Universities, Inc.